1. Field
Embodiments relate to a torque-based walking robot and a control method thereof.
2. Description of the Related Art
Recently, research and development of walking robots which have a similar joint system to that of humans and are designed to coexist with humans in human working and living spaces has been vigorously progressing. The walking robots include multi-legged walking robots having a plurality of legs, such as bipedal or tripedal walking robots, and in order to achieve stable walking of the robots, actuators, such as electric actuators and hydraulic actuators, located at respective joints need to be driven. Driving of actuators is generally divided into a position-based Zero Moment Point (ZMP) control method in which command angles, i.e., command positions, of respective joints are given and the joints are controlled so as to trace the command angles, and a torque-based Finite State Machine (FSM) control method in which command torques of respective joints are given and the joints are controlled so as to trace the command torques.
In the ZMP control method, a walking direction, a walking stride, and a walking velocity of a robot are set in advance so as to satisfy a ZMP constraint, i.e., a condition that a ZMP is present in a safety region within a support polygon formed by (a) supporting leg(s) (if the robot is supported by one leg, meaning the region of the leg, and if the robot is supported by two legs, means a region set to have a small area within a convex polygon including the regions of the two legs in consideration of safety), walking patterns of the respective legs corresponding to the set factors are created, and walking trajectories of the respective legs are calculated based on the walking patterns. Further, angles of joints of the respective legs are calculated through inverse kinematic calculation of the calculated walking trajectories, and target control values of the respective joints are calculated based on current angles and target angles of the respective joints. Further, servo control in which the respective legs trace the calculated walking trajectories per control time is achieved. That is, during walking, whether or not positions of the respective legs precisely trace the walking trajectories according to the walking patterns is detected, and if the respective legs are deviated from the walking trajectories, torques of actuators are adjusted so that the respective legs precisely trace the walking trajectories. Such a ZMP control method is a position-based control method and thus achieves precise position control, but requires precise angle control of the respective joints in order to control the ZMP and thus requires a high servo gain. Thereby, the ZMP control method requires high current, thus having low energy efficiency and high rigidity of the joints.
On the other hand, in the FSM control method, rather than tracing positions per control time, finite operating states of a robot are defined in advance, target torques of respective joints are calculated with reference to the respective operating states during walking, and the joints are controlled so as to trace the target torques. Such an FSM control method controls torques of the respective joints during walking and thus enables a low servo gain, thereby having high energy efficiency and low rigidity. Further, the FSM control method does not need to avoid kinematic singularities, thereby allowing the robot to have a more natural gait similar to that of a human.
While a robot is in motion, switching between the above-described position-based ZMP control method and torque-based FSM control method may be required. However, if switching between the control methods occurs while the robot is in motion, a current and voltage spike may occur. Due to such a current and voltage spike, sensors obtain unstable data, and thus control may be unstable and hardware may be damaged.